The present invention relates to a magnetic brush developing device for use in electrophotographic copying machines.
The magnetic brush developing device is widely used as a developing device in electrophotographic copying machines, and is constructed such that a developer comprising a thermoplastic resin colored powder (referred to as toner hereinbelow) and a magnetic carrier (referred to as carrier hereinbelow) is moved by means of a magnetic roller so as to keep the developer in sliding contact with a photosensitive member. After a period of time, the toner will deposit by fusion on the surface of the carrier, and consequently the triboelectric properties of the toner deteriorate.
As a result, a difficulty arises in which the background of the image obtained is stained black by the above-mentioned phenomenon.
This phenomenon is caused by a reduction in the static charge on the toner. To prevent the occurrence of the above-mentioned phenomenon, a developer can be used which initially has a higher charging characteristic than that actually required. If under the initial condition, however, the developer has a charging characteristic higher than that required, the reproducibility of an image with a low contrast (referred to as low contrast image) relative to the background of the original will be reduced.